The present invention relates to a screening device. More precisely, the invention refers to so-called riddles and similar screening devices, which usually consist of a support frame provided with legs, in which frame a screen deck with longitudinally extending screening apertures is fitted. The apertures are formed between longitudinally extending screening elements such as, for example, steel bars. As a rule, the deck is inclined downwards from the feed end to the discharge end to allow oversized material not passing through the apertures to be discharged at the lower end of the screen. Screening devices of this kind exist both with and without an auxiliary device in the form of a vibrator to impart a vibrating movement to the deck.
In screen decks of this kind, and particularly in the case of relatively large screening apertures (above approx. 30 mm), blocking of the deck due to oversized material wedging between the screening elements is always a problem. Therefore, several different kinds of cleaning arrangements for this type of deck have been suggested. For example, it is known to arrange every second bar pivotable relative to the other bars around a horizontal shaft at one end of the deck. Thus, every second bar can be pivoted upwards, which increases the distance to the alternating fixed bars so that accumulated material particles will either glide along the deck to its discharge end or fall down through the widened apertures. A disadvantage with the described arrangement is, however, that the movement which increases the apertures is very small near to the pivot axle. Thus, oversized or borderline stones are not freed, but only sink a little deeper into the apertures. This results in still worse jamming, which prevents the bars from being swung back and may cause deformation and damage to the deck when it is attempted to carry out this operation.